1. Technical Field
The present invention relates to a portable computer having a DC/DC converter in general, and in particular to a method for improving the efficiency of the DC/DC converter operating in a light load state.
2. Description of Related Art
A series regulator or a switching regulator can be used as a direct current (DC) regulator power supply. But switching regulators are more widely used in a notebook personal computer (notebook PC) because of their high efficiency and light weight compared to series regulators. A switching regulator controls the switching of a switching element in an operation mode, such as a Pulse Width Modulation (PCM) mode or a Pulse Frequency Modulation (PFM) mode, to produce a constant DC voltage.
Specifically, in the PWM mode, the On periods are controlled while maintaining the switching frequency that corresponds to the frequency of turning ON/OFF the switching element constant. In the PFM mode, the Off periods are controlled by maintaining the On periods constant, thereby controlling the average secondary-side voltage to be within a predetermined range when the primary-side voltage is changed. In the PWM mode and the PFM mode, the On periods and the Off periods are constant when the input voltage and the output voltage are constant. There is a limit on decreasing the lower limit of the switching frequency because it is necessary to suppress the ripple voltage or decrease the size of the elements of a smoothing circuit. Moreover, switching elements produce a loss, resulting from the switching frequency.